Personal computing systems, such as the one shown in FIG. 1A, employ an eve r increasing number and types of display devices. Those systems commonly include a central processing unit 4 which drives a video controller 6. Controller 6 interacts with a memory 8 to generate video control information. The video control information is usually coupled through a digital to analog converter 10 to drive an analog display 14 or through a buffer 12 to drive a digital display 16.
Among the possible types of displays in use are CRT and flat panel displays. Some of these devices are color and some have no color capability, i.e., are monochrome display devices. Flat panel display devices are typically monochrome devices.
It is often desirable to display an image that, even though not color, possesses some variety in shades so as to be aesthetically and functionally interesting to a user. Many personal computing systems employ gray scales to provide visual differentiation for displayed images.
Three techniques of varying sophistication are known to applicant for generating gray scales at a monochrome display. Where the display is an analog device such as a CRT, the gray scales may be generated by applying different voltage levels for the same units of time, as shown generally in FIG. 1B. Another technique, pulse width modulation (PWM), provides gray scales by varying the time for which a given constant voltage is applied to a pixel as shown in FIG. 1C. Finally, frame rate control (FRC) techniques use a plurality of frames during which the constant voltage may or may not be applied to the pixels in a display. Gray scales are achieved by applying and not applying the constant voltage to specific pixels during specific frames as shown generally in FIG. 1D.
These techniques suffer from a number of limitations when implemented. For example, when a large number of gray scales is desired, the less intense grays remain on for a relatively short period of time, which creates flicker at the display. In addition, flat panel displays are made from differing chemicals and therefore have different intensity response characteristics. This complicates the hardware and software required to provide satisfactory gray scales for a full range of flat panel displays.
Prior art patents known to Applicant neither teach or suggest any satisfactory method for resolving the foregoing problems.
U.S. Pat. No. 4, 688,031 describes the use of color masks having different repetitive dot patterns for even and odd number rows to generate gray scales corresponding to different colors.
U.S. Pat. No. 4,703,318 describes a method of forming a monochromatic image from a digital representation by replacing background and foreground colors with patterns of light and dark dots.
U.S. Pat. No. 3,845,243 describes a method and system for producing gray scales in a display. The display is divided into a number of regions equal to the number of gray scales to be produced. The number of regions excited corresponds to the gray scales produced.
U.S. Pat. No. 3,863,023 describes a technique of varying the on/off time of memory planes to generate graduations of gray corresponding to the original scanned image.
U.S. Pat. No. 3,590,156 discloses a switching system that synchronously addresses a display panel and a memory system to control the duty factor of each display element in accordance with received picture signals.
U.S. Pat. No. 4,742,346 uses a segmented shift register, with one shift register for each column of pixels in a display. Data stored in the registers is transferred to counters through multiplex circuitry. The counters control the application of voltages to the pixel columns.